The present invention concerns printers and pertains particularly to fractional dot column correction for better scan axis alignment during printing.
Thermal ink jet printers print by emitting ink droplets. The ink droplets are emitted by nozzles arranged on a printhead. Passive thermal inkjet heater circuitry is, for example, carried on a silicon chip. For general information on thermal inkjet printers, see for example, J. Stephen Aden, Jaime H. Bohorquez, Douglas M. Collins, M. Douglas Crook, Andre Garcia, and Ulrich E. Hess, The Third-Generation HP Thermal Inkjet Printhead, Hewlett-Packard Journal, February 1994, pp. 41-45.
It is possible for a thermal ink jet printer to emit discrete drops of a variable volume. For example, a pulse train of packets of pulses is used to generate drops comprising packets of connected or merged droplets. The reciprocal of the pulse repetition rate is greater than the bubble collapse time and the pulse packet rate is less than the maximum single droplet emission rate of the printhead. The individual droplets within the packet merge in flight to create a single drop whose volume depends upon the number of pulses contained within the pulse packet. See for example U.S. Pat. No. 4,503,444 issued to Christopher A. Tacklind, for METHOD AND APPARATUS FOR GENERATING A GRAY SCALE WITH A HIGH SPEED THERMAL INK JET PRINTER.
One issue when using a printhead with a large number of nozzles and a high resolution is correct alignment of all the nozzles so that the ink is correctly placed on the print media. Dot alignment can be done in both horizontal and vertical axes. The horizontal axis is generally referred to as the scan axis. The vertical axis is generally referred to as the paper advance axis (or just the paper axis).
Many factors can affect alignment. For example, slight variations during manufacturing can affect alignment. Additionally, the alignment of ink placed on a page can be dynamically affected during printing, for example dependent on the volume of a drop which is generated by merged droplets.